Studies are underway of Cytomegalovirus (CMV) growth in cells of lymphoid origin. Attempts are being made to reproduce in vitro the presumed CMV carrier state and to devise more sensitive and rapid means for virus detection. The epidemiology of CMV infection and persistence is under study, including CMV infections of the genital tract, their acquisition by sexual contact and their impact upon the fetus. We have proposed an hypothesis to explain the frequent transmission and activation of CMV in association with transfusion. This hypothesis is under test in vitro and in vivo, in clinical situations as well as in an animal model. Studies of the stimulation by CMV of cellular replication and morphology are also underway. We are also seeking to define the effect upon fetal development of interferon and interferon inducers.